Hybrid integrated circuit for a gas sensor

ABSTRACT

A hybrid integrated circuit of a gas sensor, comprising a substrate ( 1 ) in the form of a ring-shaped peripheral portion ( 2 ) and a disk-shaped central portion ( 3 ), both being interconnected through three jumpers ( 4 ) spaced 120° apart and having three branchings ( 10 ) arranged at 120° at the peripheral portion ( 2 ). The central portion ( 3 ) carries a gas-sensitive film ( 7 ), a film heater ( 6 ), and a film electrode ( 8 ) of the circuit for measuring the resistance of the gas-sensitive film ( 7 ). The heater ( 6 ) and the electrode ( 8 ) are electrically connected along the jumpers ( 4 ) to bonding pads ( 5 ) located in the peripheral portion ( 2 ). The thickness of the jumpers ( 4 ) and of the central portion ( 3 ) is 0.15-0.25 mm, and the width of the jumpers ( 4 ) is 0.05-0.15 mm.

FIELD OF THE INVENTION

The present invention relates in general to electronic engineering andmore specifically it concerns design technology for building a hybridintegrated circuit of a gas sensor.

BACKGROUND OF THE INVENTION

A thermochemical sensing transducer is known detecting the presence andconcentration of various gases in the air such as air including CH₃ orHydrogen. The known sensing transducer of prior arts provides a highermeasuring accuracy due to the layer of a catalyst (platinum, palladium,or metal oxides) applied to the external surface of the sensing element,and an extended range of gases monitored which is due to the fact thesensing elements are made of different materials, have differentdimensions, and make use of different catalysts. Two silicon stages areinstalled independently on the same substrate for two constructionallysimilar sensing elements (save the aforementioned basic differences). Aninsulating nitride layer 400 mm thick is applied to the silicon membraneof the stage. A double-meander heater is established at the center ofthe sensing element, said heater being made of silicon or nickel andhaving aluminium electric leads to the sensing element periphery. Athermocouple consisting of two meanders connected in opposition and madeof different materials is applied to the heater in perpendicular withthe axis thereof. A catalyst layer is applied from above, which may becoated with a protective gas-permeable layer. The aforecited sensingtransducer is capable of measuring concentration variations withtemperature (DE, A, 4,008,150).

However, the aforementioned construction is sophisticated andinadequately technologically manufacturable.

A structure for the gas analyzer carrier is known from prior arts, saidstructure enabling to obtain a mechanically strong construction ofanalyzers for such gases as SO₂, H₂S, and benzene suitable for thelarge-scale production. The construction of said structure appears as asquare of 6×6 mm inside which another square of 2×2 mm is placed, bothsquares being interconnected with jumpers along the four diagonalscommon to both squares. The central square and partly two oppositejumpers are coated with a layer of platinum serving as a heater. Thecentral square-shaped platinum coating is shaped as a spiral. Theresultant resistor is coated with an insulating layer. Two electrodesmade of platinum or gold are applied to the opposite sides of thecentral square, two metal strip electrodes directed along the freesubstrate jumpers being connected to said platinum or gold electrodes. Agas-sensitive film is applied to the central square between theelectrodes (FR, A, 2,625,561).

However, the construction discussed above is sophisticated difficult toassemble, has no adequate selectivity due to an insufficient heatinsulation of the central substrate portion, and is possessed ofinadequate manufacturability.

SUMMARY OF THE INVENTION

The principal object of the present invention is to provide a hybridintegrated circuit of a gas sensor having such a constructivearrangement that allows to increase the selectivity, thermal isolation,and manufacturability of the gas sensor.

The foregoing object is accomplished due to the fact that in a hybridintegrated circuit of a gas sensor, comprising a substrate in the formof a mechanically strong peripheral portion and a central portionthermally insulated therefrom, both portions being interconnectedthrough jumpers, the peripheral substrate portion is provided with filmbonding pads; while the central portion carries a film heater, agas-sensitive film, and a film electrode for connecting with a circuitfor measuring the electrical resistance of the gas-sensitive film: allthese being electrically, connected to the bonding pads through filmconductors located on the jumpers. According to the invention, theperipheral substrate portion is ring-shaped, the central portion thereofis shaped as a disk electrically connected to the peripheral portion bythree jumpers spaced 120° apart, and the jumpers have branchings at anangle of 120° at the place of connection to the peripheral portion. Thegas sensor device, as placed within an outer package, is designed tohave geometry with relative spacings between its portions such that theratio between the diameter of the central portion and the insidediameter of the peripheral portion is 0.2-0.4, and the ratio between thejumper length from the center of the circuit to the branching and theinside diameter of the peripheral portion is 0.6-0.8, while the width ofthe jumpers is 0.05-0.15 mm and the thickness of the jumpers and of thecentral substrate portion is 0.15-0.25 mm.

The film heater may be placed in a recess provided on the face surfaceof the central substrate portion.

It is desirable that the peripheral substrate portion has through holesand is fixed on a metal base of the package in such a manner that theinternal package leads are arranged in the holes of the peripheralsubstrate portion and are electrically connected to the bonding pads,while the cover of the package has a hole to admit the medium underanalyses.

Providing the ring-shaped peripheral substrate portion and thedisk-shaped central substrate portion gives an additional rigidity tothe films and circuit, integrated and reduces the stress level therein.

The interconnection of the peripheral substrate portion to the centralsubstrate portion by three jumpers spaced 120° apart is selected so asto provide the heat insulation of the central substrate portion and tocompensate optimally for any mechanical stresses arising upon heatingthe central substrate portion.

The fact that the ratio between the diameter of the central substrateportion and the inside diameter of the peripheral portion is designselected to be 0.2-0.4, and the ratio between the jumper length from thecenter of the circuit to the branching and the inside diameter of theperipheral portion to be 0.6-0.8, as well as design selection of thejumper width to be 0.05-0.15 mm and of the thickness of the jumpers andof the central substrate portion to be 0.15-0.25 is explained by thedesire to attain optimal circuit geometry for suitable thermalinsulation, according to the invention, from the viewpoint of the betterheat insulation of the central substrate portion and the strength of thefilm circuit construction.

The upper limits of the value of the aforementioned ratios (0.4 and 0.8,respectively) are dictated by the permissible value of the heatresistance of the jumpers at which the temperature of the centralsubstrate portion is within the required range for providing goodselectivity in device performance, that results because a required levelof heat insulation is provided.

The lower limits of the value of said ratios (0.2 and 0.6, respectively)are dictated by the permissible length of the jumpers at which themechanical strength of the circuit construction remains unaffected.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further illustrated by a detailed descriptionof some specific exemplary embodiments thereof to be taken withreference to the accompanying drawings, wherein:

FIG. 1 is a plan view of the filed hybrid integrated circuit of a gassensor;

FIG. 2 is a section taken on the line II—II in FIG. 1;

FIG. 3 is a view of FIG. 1 showing the respective dimensions of thecircuit elements;

FIG. 4 is a section taken on the line V—V in FIG. 3; and

FIG. 5 is a sectional view of an alternative embodiment of the filedhybrid integrated circuit of a gas sensor.

BEST METHOD OF CARRYING OUT THE INVENTION

The hybrid integrated circuit of a gas sensor for detecting CH₄, CO, andsome other gases, according to the invention, comprises a substrate 1(FIG. 1) made, e.g., of Polycor and being 0.5 mm thick, in the form of amechanically strong peripheral portion 2 (FIGS. 1 and 2) shaped as aring, e.g., 1.3 mm wide, and a central portion 3 shaped as a disk 1.2 mmin diameter thermally insulated from the peripheral portion 2. Both theperipheral portion 2 and the central portion 3 are joined up by jumpers4 (FIG. 1) made of Polikor.

The peripheral portion 2 of the substrate 1 is provided with filmbonding pads 5 having the following structure: Ti (100 Ohm/sq.mm)—Au (5μm). The central portion 3 of the substrate 1 carries a 200 Ohm 400 mWfilm heater 6 made of, e.g., a tantalum film, a gas-sensitive film 7made of, e.g., SnO₂ with catalytic additives of MgO or CaO, Pd, Au,depending on the composition of the atmosphere being monitored, and afilm electrode 8 having a structure similar to that of the bonding pad5, a circuit for measuring the resistance of the gas-sensitive film 7electrically connected to the bonding pads 5 by film conductors 9provided on the jumpers 4 spaced 120° apart, and having branchingsarranged at an angle of 120° at the place of connection to theperipheral portion 2. The ratio between the diameter of the centralportion 3 and the inside diameter of the peripheral portion 2 is0.2-0.4, and the ratio between the length of the jumper 4 from thecenter of the circuit to the branching 10 and the inside diameter of theperipheral portion 2 is 0.6-0.8 which is clearly evident from FIGS. 3and 4. The width of the jumpers 4 is 0,1 mm and the thickness of thejumpers 4 and of the central portion 3 of the substrate 1 is 0,2 mm,which is dictated by better heat insulation of the central portion 3with the circuit strength and durability remaining unaffected.

The heater 6 (FIG. 2) is established in a 10 μm deep recess 11 byfilling it with a resistive paste followed by annealing at 800° C.

The peripheral portion 2 (FIG. 5) of the substrate 1 has holes 12, e.g.,0.7 mm in diameter aligning with internal leads 13 of a package 14. Thesubstrate 1 of the hybrid integrated circuit, according to theinvention, is fixed on a metal base 15 of the package 14. The internalleads 13 of the package 14 are passed through the holes 12 in theperipheral portion 2 and are connected through a golden wire 16 tobonding pads 17 of the hybrid integrated circuit, according to theinvention. A cover 18 of the package 14 has holes 19 which are 2 mm indiameter to admit the medium under analysis, e.g., air checked forpresence of CH₄ closed by a soft net.

The circuit, according to the invention, functions as follows.

The film heater 6 (FIG. 1) delivers a supply voltage, e.g., 9 V, thusmaking possible heating the central portion 3 of the substrate 1 up to300-500° C. and maintaining the preset temperature within an accuracy of±5° C. Depending on the air impurity to be detected the temperature canbe maintained at a preset level with the aforementioned accuracy. Forinstance, at 350° C. the sensor possesses an adequate selectivity andsensitivity to hydrogen, and so on. The resistance of the gas-sensitivefilm 7 is measured in the presence and in the absence of an airimpurity, which enables a researcher to judge of the selectivity of thesensor.

Thus, the hybrid integrated circuit allows to increase the selectivitydue to a more accurate maintaining the preset temperature which is inturn due to better heat insulation of the central portion 3 of thesubstrate 1. This is attained due to an optimized circuit constructionas far as heat insulating and physical strength parameters thereof areconcerned.

Moreover, the hybrid integrated circuit is simple in construction,whereby it is less laborious in efforts of manufacturing the claimed gassensor structure.

In describing the disclosed embodiment of the present invention,specific narrow terminology is used for the sake of clarity. However,the invention is not restricted to the specific terms so selected, andit should be understood that each such term covers all equivalentelements functioning in a similar way and used for solving similarproblems.

Although the present invention has been described herein with referenceto the preferred embodiment, it will be understood that variousmodifications and alterations may occur to the details of constructionwithout departing from the spirit and scope of the invention, as will bereadily understood by those skilled in the art.

All these modifications and alterations should be considered to remainwithin the limits of the spirit and scope of the invention in accordancewith the claims that follow.

INDUSTRIAL APPLICABILITY

The present invention can be used in designing, mounting andmanufacturing a miniature gas concentration sensing transducer.

What is claimed is:
 1. A hybrid integrated circuit of a gas sensor,comprising a substrate (1) in the form of a mechanically strongperipheral portion (2) and a central disk portion (3) thermallyinsulated therefrom, both portions being interconnected through a groupof three jumpers (4) having a certain width and a special length chosenwithin a proper range, the peripheral portion (2) of the substrate (1)has a central opening characterized by an inside diameter circumferenceand is provided with a set of three film bonding pads (5), the centralportion (3) fitting within said circumference carries a film heater (6),a gas-sensitive film (7), and a film electrode (8) of a circuit formeasuring the resistance of the gas-sensitive film (7), all thesecomponents being electrically connected to the bonding pads (5) throughfilm conductors (9) located on the jumpers (4), CHARACTERIZED in thatthe peripheral portion (2) of the substrate (1) is ring-shaped, thecentral portion (3) thereof is shaped as a disk electrically connectedto the peripheral portion (2) by three jumpers (4) spaced 120° apartaround the gas sensor, the three jumpers (4) have a triad of branchings(10) arranged at an angle spacing of 120° around the gas sensor at acircular set of three places of connection to the peripheral portion(2), the ratio between the diameter of the central portion (3) and theinside diameter of the peripheral portion (2) is 0.2-0.4, and the ratiobetween the length of the jumper (4) from the center of the circuit tothe radius of the branchings (10) reaching said places of connection andthe inside diameter of the peripheral portion (2) is 0.6-0.8, while thewidth of the jumpers (4) is 0.05-0.15 mm and the thickness of thejumpers (4) and of the central portion (3) of the substrate (1) is0.15-0.25 mm.
 2. The hybrid integrated circuit of a gas sensor as setforth in claim 1, wherein said central portion has a face surface andthe film heater (6) is placed in a recess (11) provided on the facesurface of the central portion (3) of the substrate (1).
 3. The hybridintegrated circuit of a gas sensor as set forth in claim 1 wherein theperipheral portion (2) of the substrate (1) has through holes (12) andis fixed on a metal base (15) of a package (14) in such a manner thatinternal electrical leads (13) of the package (14) are arranged in theholes (12) of the peripheral portion (2) of the substrate (1) and areelectrically connected to a subset of three bonding pads (17), and acover (18) of the package (14) has holes (19) to admit the medium underanalyses.